The invention relates to a tensioning device for tensioning lines, particularly chains or belts, having a rotatable drive element serving to wind up the tensioning line, at least one ratchet wheel connected integrally in rotation thereto and exhibiting directionally oriented notches, and a drive part, serving to introduce the rotary movement into the drive element, on which a drive pawl cooperating with the ratchet wheel is mounted slidably counter to the action of a spring and which is provided with at least one control cam to transfer a blocking mechanism associated with the ratchet wheel into an inoperative position, whilst the notches of the ratchet wheel are shaped so that they are firmly retained by the detent pawl during movement of the drive part in one direction, whereas during the movement of the pivot lever in the opposite direction they are entrained by the latter via the drive pawl.
A tensioning device of the above-stated type is known from U.S. Pat. No. 4,199,182, for example. In the known tensiong device a blocking mechanism, which consists of a single detent pawl, is associated with the teeth or notches of the ratchet wheel. The number of teeth of ratchet wheels is limited by considerations of strength. It is regularly 11, which corresponds to a tooth pitch angle of approximately 33.degree.. Such a large pitch angle frequently makes it impossible to maintain even approximately the high pretensioning force which can generally be achieved per se by a drive part formed by a pivot lever, because on the one hand the engagement of the detent pawl into the next tooth gap of the ratchet wheel would necessitate a further movement of the drive part, which however cannot be managed dynamically, and on the other hand a reverse rotation of the ratchet wheel of not much less than 33.degree. in the least favorable case has to be accepted when the detent pawl engages with the tooth gap reached.